The invention relates generally to imaging systems, and more particularly to high voltage generators for imaging systems.
Computed Tomography (CT) is an X-ray medical imaging process which makes possible obtaining a three-dimensional (3D) image of a patient or object using a plurality of two-dimensional (2D) images acquired about the patient or object. In CT, dual energy imaging is known for obtaining material characterizations and/or reduction of artifacts by utilizing two scans of the patient or object at different voltage/energy levels, “low-kV” and “high-kV.”
In a single rotation around the patient or object, a high voltage generator switches between “low-kV” and “high-kV” in order to emit low energy X-rays and high energy X-rays, respectively, from an X-ray tube. The high voltage generator may typically switch, for example, between a low-kV of about 70 to 100 kilovolts (kV) and a high-kV of about 120 to 150 kV. The low energy and high energy X-rays emitted, after being attenuated by the patient or object, impinge upon an array of radiation detectors. The intensity of the X-rays may then be processed to produce an image.
To complete a dual energy scan in a single rotation, the high voltage generator must rapidly switch between low-kV and high-kV. Such rapid switching may typically be performed between 10 μs and 30 μs, though even faster times may be desirable. However, the high voltage generator typically includes a high voltage (HV) capacitance which may include a filtering capacitor and/or parasitic capacitance (such as from high voltage cabling). As a result, the fall time between high-kV and low-kV is related to the discharge of the HV capacitance. In a typical CT system, the tube current may oftentimes be the largest part of the current resulting from discharge of the HV capacitance.
In CT, it is also often desirable to modulate tube current supplied by the high voltage generator in order to adjust the X-ray exposure for different parts of the body or differently sized objects. This helps to prevent overexposing or underexposing the patient or object during data acquisition.
However, modulating tube current during a dual energy scan, such as during a high-kV time, creates different (inconsistent) fall times between high-kV and low-kV from cycle to cycle. This, in turn, creates undesirable disproportionate energy separation between the energy amounts transferred during high-kV times and the energy amounts transferred during low-kV times. In other words, modulating tube current during dual energy scans may result in non-ideal waveforms which may impact the ability to effectively reconstruct scanned images.
In addition, the HV capacitance may vary in time, for example, as a function of temperature. This may also create different, inconsistent fall times, particularly as compared to any calibration which may have been done at a different temperature.
Therefore, it is desirable to provide an improved high voltage generator which provides a generally constant energy separation between high-kV and low-kV times during a dual energy scan in which tube current modulation is used. Moreover, it is desirable to provide an improved high voltage generator which may provide substantially identical energy separation between high-kV and low-kV times during a dual energy scan executed under differing environmental conditions.